Devices of the membrane MEMS type especially include pressure transducers which are intended to be immersed into a liquid or gas immersion medium. The immersion medium dictates its pressure to the surface of a suspended membrane. The latter deforms under the action of this pressure. The measured value of this deformation is representative of the value of the pressure applied. The deformation measurement can be made:                either by capacitive effect. In this case, the deformable membrane is one of the armatures of an electrical capacitor. The deformation modifies the capacitance value, and the capacitance variation is electrically measured.        or by piezoelectric effect. In this case, piezoelectric elements are mechanically associated with the membrane, so that the deformation thereof is representative of the deformation of the membrane. Deformation of the piezoelectric elements results in an electric potential difference occurring between two points of the piezoelectric element. This potential difference is measurable, which enables the deformation of the membrane and the pressure applied to be known.        
Surfaces of MEMSs which are in contact with the external medium have sometimes to be unidirectional or bidirectional hermetic barriers, whether to protect the external medium from contaminations related to materials making up the MEMS or to protect the MEMS from aggressions of the external medium. Thus, a protective barrier enabling the external medium to be protected from contaminations related to the MEMS is for example necessary when the MEMS is introduced into a biological medium. A protective barrier can also be necessary when the MEMS is introduced into a harsh or corrosive medium. A hermetic protective barrier can also be necessary to keep a controlled atmosphere inside the MEMS, for example in case of gas diffusion.
To do so, it is known to use protective coatings on the membranes of the MEMSs. Prior art coatings can be metals, oxides, nitrogen or carbon compounds or even polymers. Choice criteria are chemical inertness, biocompatibility, hermeticity, electromagnetic transparency for communicating devices, and deposition techniques.
However, the coating should not modify rigidity of the movable membrane of the MEMS. But it is difficult to find a coating which does not modify the rigidity of the membrane of the MEMS, and which has in the meantime the necessary characteristics in terms of chemical inertness, biocompatibility and hermeticity.